Industrial-rated circuit breakers such as described within U.S. Pat. No. 4,782,583 entitled "Method of Assembly of a Molded Case Circuit Breaker Crossbar" require operating components that are sized to handle the largest available current during circuit interruption. Since a short circuit available current is only limited by the residual circuit impedance, the circuit breakers within industrial environments are sized to handle short circuit currents which are several orders of magnitude greater than faults occurring because of routine circuit faults that are designated as "long time" or "short time" faults to distinguish from the short circuit faults.
Several attempts have been made to limit the size of the circuit breaker current-carrying components by limiting the short circuit current to a manageable level. One such attempt is in the "arcless" circuit interrupter described within U.S. Pat. No. 4,583,146 entitled "Fault Current Arcless Circuit Interruption" wherein the current is transferred to a voltage variable resistor before contact separation to eliminate the arc that occurs upon contact separation and to rapidly separate the contacts during the early stages of the current waveform.
U.S. Pat. No. 4,375,021 entitled "Rapid Electric Arc Extinguishing" describes the use of so-called "blow open" contact arms to limit the circuit overload current to a reasonable value prior to the articulation of the circuit breaker operating mechanism to interrupt the circuit current.
U.S. Pat. No. 3,488,761 entitled "Current Limiter Device" describes the combination of a positive temperature coefficient material (PTC) in parallel with a metal resistor and in series with a pair of separable contacts to limit the current that is interrupted by the contacts.
U.S. Pat. No. 4,413,301 entitled "Circuit Protection Devices" describes the use of a PTC device in series with a metallic resistor and the separable contacts.
One object of the present invention is to describe a current limiter unit employing a PTC device to assist in limiting short-circuit current to a predetermined value to allow circuit breakers of low short-circuit ratings to be employed within higher rated electrical distribution circuits. Another object of the present invention is to describe a current limiter unit that avoids expenditure of electric power during quiescent current conditions.